Isolate object with / numpad key Blender 2.8
The / key is generating a Local (isolated) view of only the selected object, while that in a sense hides other objects, it is more like a isolation chamber rather then hiding all else and then later making things visible again (in the outliner you will see that the other objects are not being hidden).
In Blender 2.7, using Background images managed in the Properties panel opened with N, found at the bottom, it was possible to have the references still visible in Local mode, as those were not managed as objects, but bound to the view.

Blender 2.8+ deals with Background images now by using empties to hold the pictures. That makes them objects, which in turn if not selected will be locked out when you have another object selected and go in Local mode by pressing /.

Solution:
Keeping the background images visible is possible by simply selecting the images, or their corresponding empties, as well as the object you want to go in Local mode with. All selected objects will be taken in Local mode and the images will stay visible.